Income Taxes (Details)	9 Months Ended
Sep. 30, 2022
Effect of Tax Cuts and Jobs Act of 2017 1 [Abstract]
Capitalize Research And Development Expenses, Amortization Period	5 years
Capitalize Research And Experimentation Expenses, Amortization Period	15 years

Amortization period for capitalize research and experimentation expenses.

No definition available.

Name:	imgn_CapitalizeResearchAndExperimentationExpensesAmortizationPeriod
Namespace Prefix:	imgn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration